Method of hot dip galvanizing a ferrous metal



METHOD OF HOT DIP GALVANIZING A FERROUS METAL Filed Feb. 19, 1959 Ferrous Metal Article to be Galvanized Surface Cleaning Treatment Wet with Aqueous Ammonium Chloride Zinc Chloride Solution (lO%-75% NH Cl+90%-25%ZnCl Drying Dry Fluxed Article Introduce dry fluxed article through molten Galvanized flux cover Sol? (40%- rti le 80% Zn Gl l5%-45% KCl, up to 20% other halides) I into molten Zinc INVENTOR Allen T. Baldwin ATTORNEYS United States Patent bfifice 2,940,870 Patented June 14, 1960 METHOD OF HOT DIP GALVANIZIN G A FERROUS LIETAL 7 Filed Feb. 19, 1959, Set. No. 794,234 I 7 Claims. Cl. 117-52 This invention relates to galvanizing ferrous metal articles by the hot dip method, and is particularly directed to the provision of an improved procedure for fluxing the ferrous metal to insure formation thereon of a continuous, tightly adherent film of zinc. The process of the invention involves wetting the ferrous metal with an aqueous solution of a flux of ammoniacal zinc chloride and drying the solution to form a thin deposit of the flux on the article, after which the article is dipped in a molten bath of zinc covered by a fluid salt blanket which is at all times essentially free of ammonia (including ammonium salts).

Hot dip galvanizing of iron and steel to protect the ferrous metal from corrosion has been'practiced on a large scale for more than a century. The procedure has always entailed fluxing the ferrous metal with a salt or salt mixture to clean it of oxides and other surface contaminants, so that the zinc may Wet it effectively, and then immersing the fluxed article in a bath of molten line. The flux that has been most extensively used, and probably for the longest time, is zinc ammonium chloride. Traditionally, in hot dip galvanizing operations, the zinc bath has been maintained molten in a heated steel pot and has been covered with a fused layer of a molten salt flux consisting solely, or composed in large part, of zinc and ammonium chlorides (or zinc ammonium chloride). The long continued use of this flux is owing to its extensively effective cleaning action on the ferrous metal surface, which makes for easy formation of a continuous, adherent film of zinc, substantially free of pin holes and other imperfections.

Despite this prime virtue, however, the use of an ammoniacal flux in hot dip galvanizing is not without'its serious disadvantages. The very effectiveness of the salt as a flux is owing to its rapid attack on the ferrous metal surface. This attack unfortunately does not stop with preparation of the metal surfaces for receiving the zinc coating-it promotes the formation of zinc-iron alloy interfacial layers of undesirably great thickness if the time of the galvanizing operation is not carefully controlled. Furthermore, a molten flux blanket composed to a substantial extent of ammonium chloride floating on the zinc bath undergoes a significant change in composition with continued use. It loses ammonia, in part by volatization of the ammonium chloride itself and in part by volatilization of reaction products formed by its attack on the ferrous metal or on constituents (such as aluminum) of the zinc bath; and it accumulates iron. Such composition changes always lead to decreased fluidity of the flux and decreased effectiveness in its fluxing properties. It is periodically necessary to skim off the old flux blanket and replace it with a fresh melt of the flux salts. Changing the flux blanket necessitates discontinuing for the time being the galvanizing operation. Interruptions of this sort have become increasingly objectionable with increasing mechanization of the handling of the ferrous articleslthrough the galvanizing operation.

=,Because of these disadvantages, considerable effort has been devoted'to finding a vfluxing procedure which will assure close control over the chemical attack of the flux on the metal surface, and will at the same time minimize the costly and time consuming skimming of old flux and replacement of it with new on the surface of the zinc bath. The only substantial advance heretofore made toward achieving these objectives has been the development of the so-called dry'galvanizing process- This process involves immersing the ferrous article in an aqueous solution ofammoniacal zinc chloride, and drying the article to leave a thin deposit of the flux on the surface of the metal. The article is immersed in the molten zinc after it has been dried (whence the name dry galvanizing, to distinguish this process from one in which the article is wetted with molten flux before being immersed in the molten zinc, a procedure called wet galvanizing). No flux, in the conventional sense, is employed on the surface of the zinc bath in dry galvanizing, for the residue of the salt fihn on the surface of the article as it is im mersed in the zinc is relied on to activate the ferrous metal surface for reception of the zinc. There is, however, an accumulation of ashes or spent flux or both which accrueson the surface of the molten metal and which must be periodically skimmed ofl. Hence dry galvanizing, while it reduces substantially the adverse efifects of uncontrolled fluxing of the ferrous metal surfaces and excessive loss of aluminum from the zinc bath, does not avoid the need for periodic skimming, and it increases the difficulty of maintaining a good clean and fluid surface on the zinc bath,

i The present invention provides an improved galvanizing process which combines the best features of dry and wet galvanizing while elirninating largely the disadvantages of both. The process of the invention entails fluxing the ferrous metal article substantially by the procedure of dry galvanizing, using zinc ammonium chloride as the active flux salt, and maintaining a fluid protective blanket ofan ammonia-free salt mixture on the bath of molten zinc. The salt blanket provides a thinly fluid protective cover on the molten zinc; and in accordance with the invention it is substantially free of ammonia (or any ammonium salt) and has the property of remaining free of ammonia by favoring volatilization of such ammonia as is introduced into it by the flux-coated ferrous metal articles. Thus the salt blanket is of stable composition which retains its fluidity for prolonged periodsof time, and need only be replenished from time to time by fresh additions of the same composition to make up for dragout or other losses. By use of the process of the invention, interruption of the galvanizing operation to skim the surface of the molten zinc is eliminated.

' Thenew galvanizing method comprises wetting the 1 ferrous metal article with an aqueous ammoniacal zinc T or '2% 'It is therefore a highly chlorides solution, then drying the article (either at room temperature or by heating to a temperature which may range up to 400 F. or more) and then introducing the dried article into a bath of molten zinc through a molten salt blanket floating thereon. The salt blanket is composed predominantly of zinc chloride (constituting 40% to by weight of the blanket) and potassium chloride (constituting 15% to 45% by weight of the blanket), and it preferably in addition contains other halides (especially chlorides) in amounts up to 20% by weight; its melting point should be well below 700' F., and it should be thinly fluid at that temperature. The salt blanket as pre pared is substantially free of ammonia (and ammonium salts), and remains substantially Ammonia compounds introduced into the salt blanket by introduction of the flux-coated ferrous metal articles rapidly are dissipated by volatilization, so that even after' aprolonged period of use the blanket contains only a very small quantity of ammonia (generally not over a 1% which retains its high degree of fluidity and protective free thereof during use.

stable composition i ii a be through the several s the articles through tth e'fgalv to interrupt o'perat i 'a sh'cs from. the bath. '1

' metal are most' ejasily ip such purpose. i

' article to b sflmnized In gene it h ate quality indefinitely. It is only'necessary to add make lp amounts of the same salt composition from time to time, without havingto skim the bath" at any time: The new galvanizing method may be carried out either asan intermittent operation inwhich the article to be i QQI 15. 3 continuous operation in hi'chltlie a'n isfla' ely'o'r mechanized. It is on 'of he tionii however, that it perin t a r ti in e i ia l V cr'e'te'articl'essmh as 'p'aiil s,- i anfsft'a'nks'jandother'h ware items are most'cofvenintly handled on a batch basis, especially if their nilrhbjer is notlarge and does' no t justify installation 'offa suitable conveyor system. v

The accompanying flow sheet schematically shows the 2 sequence of steps'employed in carrying out aipreferred embodiment of the invention, which ei nbodimentfis described below in considerable detail. V

v 'e articles to be""galv'ani'zed are first thoroughly 7 cleaned. The invention contemplates thesamefso'rt 'of cleaning that new-ls practicediii conventional galvaniz ing operations. lt'involves usually a thorough degr ezasing in ei her or, both a liquid or'v'apor degreasing chamber arid" an cleaning Sdllitibil, and the fih jlickling OPHQtiOiixiEcid. '1 sb' ti' a f ercl a n the art le iswg tg d with the mmoniacar iiiidchloride fiui'solutioin, 7 operation can be carried .outinany convenient w y; E01- example, the -rifles maybe dippedin' a hath'dfthe flu; solution, of the solution may be applied bysprayin'g it 'on the an les. 'The flux solution itself is "primarily: an ueens 'solution'of zinc and ammonium chloridesfl These salts are brjeferably used in the proportions of 10% to 75% ammonium chloride byfiveight of the mixture of salts and" 9 0%"to 25% zinc chloride by weight of the mix-i 4t ture Preferablythesejsaltsare' dissolved in water tol a concentrationof froni' /z to3 pounds of the'mixture'ilei; ga g V w 7..-, 1 a 7 "Instead of; using a mixture of'zinc and ammonium 1 d (Z, C z lY- 4 qli 1Y fi$ 9 TBS may beiebtained f sin a zine am onium chloride (.ei

t ff iri e h sbliittefi nes ing as 9' c tains a c eams ns, agent suc sl Y' zine, 1

asoluble igluco'sid'e'or carbohydrate, tsusw emulsion, or.

' advantage bfe'p'resent i'n theffiux solu'tion,'to insure that, 7

it efiectivelywets all parts of thearticle to be galvanized. 3

Al conventional wettjg agentfmay be employed for y a -bf. xamples]: queou solution on n ng 2 Ro nder 'g llgn'l fl g a s e t 9 it compos d r wwsi c 'a mo um 'ch %f'by weight of z inc chloride, about 1% ounces per gallon "(11 grams :per'literl of glycerine, and about elvs l flz ams. P i e bi a. we t n a nt such assorbitan' monopalmitate, is a very. satistactory, ux sq' nti r To "Mo t, ommo ly. hc x; olu on pli d-hot w the v ntur a ove 9?.- En p t say, 1,999 It is d all'alkal :b'co nie heated suhst'antially to the temperature of the 7 made 3; of aresiduepf .thezinc and ammonium chloi es aud'oi suchifreaction products as have formed in 9 lunce oi the'attaclg of the flux on .the surface of thenls al- V r lhe dried article should be immersed -.in the molten galvanizing both as promptly as is practical after It dfifimblcio'minimize exposure of the article gs t9 the air'after jdrying and before dipping in the molten zinc bath to conserve the heat of the dried article and to min mize {the v ogr'iortnnity for atmospheric oxidation or other attaglgon itsfinx-coa'led surface. Immersion involves'passing the article through the molten salt'blanket co tering the molten zinc, to beneaththe zinc surface and hgnw'thdmuin thearticlc againafromathe zinc bath.

1 g g rglng article is coated with a thin, adherent filmoj zinc. The article is cooled. to room temneratnre-, in any desired, manner, advantageously by Q2}?! in wit wat a 7 11g accordance vzith the invention, the molten salt blanket floated on the molten zinc is initially. prepared free f om am oma nd ammonium salts, and in fact c'n on rtx of favoring elimination of mm y vol ll tiqrt. "lllisjsalt bla kct is composed predomi- I 93973). by wcightof'zinc chlorideand 151% to 45% by weight ofzpotassium chloride.

Advantageonslyit also 'containsllp to 20% by weight of othe halides, particularly chlorides of 'the alkali and alkali. l mb/metals. Very satisfactory'salt blankets are cogmofiibf i595% "to; .7 5%,;-hy weig'htof zinc chloride,

t0; 97 y. weight. Qiwthcfhalides, 'eg. 5%. to 15% by wei ht :oifsodinm chloride: and up-to 10% byFwjeight of .e'arthinet'al 'chloride'rsu'ch as calcium chloride ratagnesiunt chloride. Within this composition'range, spec lly atisfactory [results shave been attained with allrnt itt res 'S QII P05 1,. J.fi',55%-.:tO by weight of zinc chlQI-fiaifith to 35% by weightof potasium chlorislcsi I 5% 1 to; 12%, by weight of sodium chloride, and 3% to 7 fbyweight oiealcium'chlorme Zinc i lqride is t e mairl'ingredient of the salt blanket, is combined with 'potasiuinchloride 'in proportions uchjas to. yield a "mixture which is completely molten fil tblulygflllid at .aZ-temperature well below 700 F. P9i3$ium ch ride-is employed both because in com- V able-tor similar t ammnna also ts help lower the melt-l ing point and the'viscositv when molten of; the mixtute ton; I I V d M wax-bended inso 'e'iases advantage in the.

hichf'sodiumkhloride has. been added. Other salts .QII chlorideit yields "a low melting mix-' mixture, either in addition to or as substitutes for the calcium chloride and for a part or all of the sodium chloride, include magnesium chloride, sodium aluminum fluoride, tin chloride, barium chloride, and strontium chloride.

The salt mixture should be thinly fluid at the temperature of the zinc bath on which it floats as a covering blanket, and it should have a low, watery viscosity at such temperature so as to flow readily over the entire surface of the zinc bath and to drain quickly and quite completely from the ferrous metal articles withdrawn from the molten zinc. To this end the flux blanket preferably is composed of salts in such proportions as to yield a mixture which is completely molten at 650 F., and preferably at 600 F. or lower. It is possible, with salt mixtures of the character contemplated, to attain melting points (temperature at which the mixture is completely molten) below 500 F.

In addition to low melting point and low viscosity, the salt mixture should possess a high degree of stability at the galvanizing temperature. It should not exert a corrosive or other undesirable attack on either the ferrous metal articles being galvanized, or on the molten zinc or any component of the zinc bath. It should not tend to change in composition during use to the extent that it increases substantially in melting point or viscosity. Hence it should neither tend to accumulate substantial amounts of foreign salts or other compounds from the zinc or ferrous metal, and should not tend to decompose or lose any of its ingredients preferentially.

An example of a particularly satisfactory salt mixture which melts at a temperature below 500 F. and possesses in a high degree the above mentioned properties of low viscosity and high stability at the galvanizing temperature, is one composed nominally of 60% by weight of zinc chloride, 25% by weight of potassium chloride, by weight of sodium chloride, and 5% by weight of calcium chloride.

The salt mixture may be deposited in solid form on the surface of the molten zinc, where it is melted by the heat of the zinc. The zinc bath is heated to about 825 F. or 850 F. (a normal galvanizing temperature), during galvanizing and the salt mixture is fused and heated to such temperature by the heat of the molten metal. The fused salt, being of low viscosity, spreads uniformly over the entire surface of the molten metal. Preferably enough is employed to cover the surface of the zinc with a layer about one-half inch, more or less, in thickness.

When the dry fluxed ferrous metal is introduced through the fused salt blanket into the molten zinc, the residue of ammoniacal compound on the ferrous surface fuses and mixes with the salt blanket. However, it is quite rapidly expelled again, by volatilization, at the temperature at which the salt blanket is maintained during galvanizing. Consequently it does not cause any significant change to occur in the composition of the salt blanket. In particular it does not accumulate therein in sufiicient quantity to cause the blanket to react undesirably with the molten zinc or any other component (such as aluminum) of the galvanizing baill or to cause it to attack the ferrous metal or promote an excessive thickness of iron-zinc alloy at the interface of the ferrous metal and the zinc coating. The particular salt blanket composition mentioned above by way of example will not generally accumulate more than about 0.25% by weight of ammonia even after prolonged use in a galvanizing operation according to this invention.

Some iron does enter the fused salt blanket, by reason of fusion therein of ferrous reaction products of the ammoniacal-fiux with the ferrous metal article. It is quickly reduced by reaction with the molten zinc, however, and collects in the dross which forms during operation of the process.

The ferrous metal is held immersed in the molten zinc only long enough to be wetted by it, and is then withdrawn again from the zinc bath. It is preferable practice to withdraw the articles from the bath at a point when the bath is free of molten salt, in order to avoid picking up salt on the emerging zinc-coated article. To this end a partition may extend across the surface of the zinc bath, to confine the molten salt blanket to that part of the bath when the articles being coated are introduced, and to exclude it from the surface of the bath in the vicinity Where the articles are withdrawn. In some cases such partition may be omitted, and the surface of the zinc bath may be kept free of molten salt in the region where the articles emerge by skimming. In other cases special forms of galvanizing pots may be provided to permit withdrawal of the articles without bringing them in contact with the molten salt blanket. In yet other cases, especially where the molten flux has a low melting temperature and a low viscosity, it may be satisfactory to withdraw the articles through the salt bracket, and to Wash oif any thin film of salt adhering to them.

It is desirable to cool the articles promptly after they emerge from the zinc bath, to prevent undue growth of the iron-zinc alloy at the interface of the zinc coating and the basic metal. Cooling advantageously is by a water quench, either in a pool of water or by a water spray.

It is undesirable to hold the articles in the quench water longer than is necessary to cool it to a temperatures low enough to substantially stop further development of the interfacial alloy. The quenched articles should still be hot enough to dry rapidly by evaporation of the water adhering to its surface. Often for this reason, it is preferred to use hot water (at a temperature from 120 F. to 200 F.) for quenching.

If the galvanized article has been withdrawn from the molten zinc bath through the fused salt blanket, salt adhering to it may be washed off by the quench water. Salt removed from the bath cover in this manner (socalled drag-out) cannot conveniently be recovered. It is replaced from time to time by simply adding make-up amounts of the original salt mixture to the surface of the zinc bath.

The zinc coating on properly cleaned articles galvanized by the method of this invention is of excellent quality. It is of uniform thickness, tightly adherent to the underlying metal, substantially free of pin holes, and with a desirably thin layer of iron-zinc alloy at the interface between the ferrous metal and the zinc coating. Control of the attack of the ammoniacal flux on the ferrous metal is readily achieved by controlling the concentration and relative proportions of zinc and ammonium chlorides in the flux solution, and by controlling the temperature and duration of the drying operation. The process is admirably adapted to being highly mechanized, for the step of coating with flux solution, drying, and carrying through the galvanizing bath can all be performed as the articles being galvanized are being conveyed on a mechanical conveyor system. The stability and low viscosity of the fused salt blanket which covers the molten zinc bath eliminates the need for periodic skimming of the bath. Owing to some or all of these desiiable features, the new process may be used with advantage in almost all galvanizing operations.

I claim:

1. The method of galvanizing a ferrous metal article which comprises wetting the article with an aqueous ammoniacal zinc chloride solution, drying the article, introducing the dried article into a bath of molten zinc through a molten salt blanket floating thereon, said salt l'anket being composed predominantly of 40% to by weight of zinc chloride and 15% to 45% by weight of potassium chloride, said salt blanket having a melting point below 700 F. and being and remaining substantially free of ammonia, and withdrawing the article with a thin adherent coating of zinc from the zinc bath.

2. The method of galvanizing a ferrous metal article containing from 10% t o75,% lily weight chloridefdry'ng .the article to leave a thin residue of thereon introducing the dried article int o a bath of molten zinc through a molten salt blanket floating" thereon, said salt blanket consisting essentially o f 5 0% 't'o 75% by'weight of zinc chloride, 15% .to 4 5% 'by weight of'potass inm lemmas, and new 20% y weight of other-"halides of "e'alkali a'ndalkali earth metals, said salt blanket haying a melting point below 650 F. and being and remaining substantially free of ammonia, and withdrawing the article with a :thin adherent coating of zinc from the 'zinc bath. 7

3. The method of tgalyanizing a ferrous metal article which comprises wetting the article with an aqueous ammoni acal Zinc chloride solution, drying .the article, introducing the dried article into a bath of molten zinc through a molten salt blanket floating-thereon, said salt blanket consisting essentially of -55% to 65% by Weight of zinc chloride, 15% to 35% by weight of potassium chloride, 5% to 15% by weight of sodium chloride, and up to 10% by weight of analkali earth metal chloride, said salt blanket having a melting point'below 600? F. and being and remaining substantially'tfree of ammonia, and withdrawing the article with a thin adherent coating of a zinc from the'zinc bath.

y 4. The method of galvaniiing a'ferrous metal article which comprises wetting the article with an aqueous solution of a flux mixture 'of'z'inc and ammonium chlofides containing'from .10% to 75% by weight of 21m.-

mes-mm chloride, drying the article to leave a thin re "due of flux thereon; introducing the dried article into a bath of molten zinc through a molten salt blanketfloating thereon, said salt blanket containing approximately 60% by'weig'ht of zincchloride, approximately 25% by weight of potassium chloride, approximately 10% by weight of sodium chloride; and approximately 5% by weight of calcium chloride and having a melting point below 5095' R, said salt blanket being and remaining substantially free of ammonia, and withdra'wingthe article with a thin adherent coating of zinc from the zinc bath. a 1

5. The method of galvanizing a ferrous metal article which comprises immersing the article in an aqueous so lution containing from /2 to 3 pounds per gallon of a rniJgture comprising from 10% to 75% by weight of ammonium chloride and the balance essentially all inc wchloride, drying the tarticle fintroducing the dried article n z' a i iu aael mas umahlqns v melting point below 650 F., said 'salt being and in s b ant al y :i e o am onia; :agd Twhh main- .;!g the the zinc article withlta adherent coating f zine trom matin 1 H n eth d-gf g lvan ugaiie rou zme al r iel w c 9Pl 3! l?g th --fi =le, I im in he cleaned article in anaqueous ammoniacal zinc chloride solution, heated toa temperature above 120 -F.-until the article heated substantially =10 :the temperature of the solution, .withdrawing the -airticle from the solution and drying it I substantially completely, introducing the dried article into a -bath of molten-zine through a -molten salt l l e s fl at n itll wx ss d s -blan e be ng.compo ed predominantly of to 80%iby weight of zinc'ehloride and 15% .to by weight ofpotassiurmchloride,said salt fblflllkflt 'haying ;.amelting-poi-nt below 700 and being and remainin bst nt a 1;y tr o amm n and withdr awirigthe article with a adherent coating of zinc from -the-zinc V 7. ;The :method of :galvanizing I a ferrous -metal article which comprises immersing the article in an aqueous solution of aflux 3 mixture or and ammonium chlorides c n ai in 20m -1;0% t 5% by ei h ofammonium chlorideheated toa temperature above 120 -F. until the a l i fi l-v .bi tiel y to th emperature of the solution, thd win h attis eef om t solu ion and a win i mg s bstant a y omp e e y dr d; i z mdl sin th r fl d-a cle nto- 1 -:ba.th of m l en zin -,h ough a umlten 2 9 g lanket :flOatiQg ther on; -s i .sal blanket consisting essentially of 59% (to "by Weight :of zinc chloride, 15% to 415% by weight of potassium chloride and- 11p to 2 0%' by weight of other halides of the e a nd k l s hsmet lelsa q alt bl nk t i ha i a melting point below 650 'F. and being and remainingsubstantially iree of ammonia, and withdrawing 41. 5 :article adherentgcoating of zinc from thezingbath.

References gcjted in therfile of this {Satent UNITED STATES PATENTS I l- 12 s P ,2 11 3.9,

h j an... 

1. THE METHOD OF GALVANIZING A FERROUS METAL ARTICLE WHICH COMPRISES WETTING THE ARTICLE WITH AN AQUEOUS AMMONIACAL ZINC CHLORIDE SOLUTION, DRYING THE ARTICLE, INTRODUCING THE DRIED ARTICLE INTO A BATH OF MOLTEN ZINC THROUGH A MOLTEN SALT BLANKET FLOATING THEREON, SAID SALT BLANKET BEING COMPOSED PREDOMINANTLY OF 40% TO 80% BY WEIGHT OF ZINC CHLORIDE AND 15% TO 45% BY WEIGHT OF POTASSIUM CHLORIDE, SAID SALT BLANKET HAVING A MELTING POINT BELOW 700*F. AND BEING AND REMAINING SUBSTAN- 